PPPL Physicists Made a Major Advance in the Field of Energy Development Globally

Stellarators, twisty magnetic devices that aim to harness on Earth the fusion energy that powers the sun and stars, have long played second fiddle to more widely used doughnut-shaped facilities known as tokamaks.

The complex twisted stellarator magnets have been difficult to design and have previously allowed greater leakage of the superhigh heat from fusion reactions.

Now Scientists at the Max Planck Institute for Plasma Physics (IPP), working in collaboration with researchers that include the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), have shown that the Wendelstein 7-X (W7-X) device inlink Greifswald, Germany, the largest and most advanced stellarator in the world, is capable of confining heat that reaches temperatures twice as great as the core of the sun.

A recent report on W7-X findings in Nature magazine confirms the success of the efforts of designers to shape the intricately twisted stellarator magnets to reduce neoclassical transport.

First author of the paper was physicist Craig Beidler of the IPP Theory Division. “It’s really exciting news for fusion that this design has been successful,” said Pablant, a coauthor along with Langenberg of the paper. “It clearly shows that this kind of optimization can be done.”

The power of fusion is at the center of energy development efforts around the world. In theory, it relies on harnessing the energy released when cores fuse in plasma to produce a heavier element: the same process that occurs in the cores of stars. And if we can achieve this, the benefits will be enormous – clean, high-output energy that is virtually inexhaustible.

However, that is easier said than done. Fusion is a very active process, and it is not easy to contain. Fusion energy was first researched in the 1940s; Decades later, fusion reactors still don’t produce as much energy as they lose, by a very large margin — although the gap is narrowing.

The fusion technology that breaks temperature records is now known as a “tokamak” – a doughnut-shaped ring of plasma trapped in an envelope of magnetic fields, propelled at high speed in rapid pulses.

Stellarators, on the other hand, rely on an incredibly complex configuration of magnets identified by an AI that can direct plasma to keep it flowing. They are so difficult to design and build, that stellarators leak a great deal of the energy from fusion, in the form of heat loss, the result of a process called neoclassical transport, where ion collisions in a fusion reactor cause plasma to spread outward.

Because tokamak has its own shortcomings, researchers at PPPL and the Max Planck Institute for Plasma Physics sought to form magnets in W7-X to try to reduce the effects of neoclassical transport. Now, measurements, taken using an instrument called an X-ray Imaging Crystal Spectrometer (XICS), have shown very high temperatures inside the reactor.

It is supported by charge exchange recombination spectroscopy (CXRS) measurements, which are believed to be more accurate than XICS measurements, but cannot be taken under all conditions.

But with the two data sets in agreement, it appears that the star was able to achieve temperatures close to 30 million Kelvin.

The team found that this would only be possible if there was a sharp drop in neoclassical transmission. They ran modeling to determine how much heat would be lost via neoclassical transport if W7-X had not been improved, and found that 30 million K was a long way off.

“This showed that the optimized shape of W7-X reduced the neoclassical transport and was necessary for the performance seen in W7-X experiments,” Pablant said. “It was a way of showing how important the optimization was.”

The results mark a step toward enabling stellarators based on the W7-X design to lead to a practical fusion reactor, he added. “But reducing neoclassical transport isn’t the only thing you have to do. There are a whole bunch of other goals that have to be shown, including running steady and reducing the turbulent transport.” Producing turbulent transport are ripples and eddies that run through the plasma as the second main source of heat loss.

With various nuclear fusion reactor technologies currently under development, it seems only a matter of time before one of them is delivered. It may take a while for the energy from fusion to reach our power grids, but when it does, it could change the world.

The W7-X is currently undergoing upgrades and will resume operations in 2022.

The research was published in the journal Nature.